comparing acids (dilute and strong)

7/27/2019 Comparing acids (dilute and strong)

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Comparing

Concentrated and Dilute Acids

F.4 Chemistry



Physical Property

Characteristic: No new substance will be formed in the

testing process

Examples: pH value, melting point, boiling point, density,

colour, magnetic property, conductivity, solubility …..



Chemical Property

Characteristic: new Substance(s) will be formed in the

testing process.

Examples: Chemical reactions with other reagents., i.e.

reaction of acids with metals to form hydrogen.



Physical Properties

1. pH Paper pH paper

Dilute HCl(aq) Conc. HCl(aq)

Reason: related to the concentration of H +(aq)

After dipping



2. Electrical Conductivity

Dilute HCl(aq) Conc. HCl(aq)

dim bright

Reason: related to the concentration of ions present



Chemical Properties

1. Reaction with carbonates

• Same for all mineral acids, dilute or concentrated

• Observations

•Gas bubbles evolve

•Solids dissolve

Reason: Concentrated acids have ahigher concentration of Hydrogenion so they react faster and bubblesare given out at a faster rate

carbonateCO 3

2-

+ 2H+

H2O + CO 2

CO 2



Chemical Properties

2. Reaction with oxides and hydroxides e.g. copper(II) oxide

• same for all acids(dilute or concentrated)

• Observationblack powder dissolvesa green/blue solution remains

Reason: black copper(II) oxidedissolves to form Cu 2+(aq),which is blue/green in colour. Before After

Copper(II) oxide

CuO (s) + 2HCl (aq) CuCl 2 (aq) + H 2O (l)



Chemical Properties

3. Reaction with metals

Dilute Concentrated

HydrochloricAcid Gives HydrogenGas Gives HydrogenGasSulphuric Acid Gives Hydrogen

GasGives Sulphur Dioxide Gas

Nitric Acid Gives NitrogenMonoxide Gas

Gives NitrogenDioxide Gas



Observations

1. Hydrogen GasPop

• Colourless gas evolves

• Gives a ‘pop’ sound with a burning splint

• The metal dissolves

N.B.: Not for metals below lead in the Reactivity Series

HCl(aq)

Zinc metal

Zn (aq) + 2 HCl (aq) ZnCl 2 (aq) + H 2 (aq)

Video



Observations

2. Sulphur Dioxide Gas

• Colourless gas evolves


N.B.: The reaction is faster when hot acid is used.

H2SO

4(l)

Zinc metal

K 2Cr 2O7/H+(aq)

Zn (s) + 2 H 2SO 4 (l) ZnSO 4 (aq) + SO 2 (g) + 2H 2O (l)

• It turns orange potassium dichromate paper green



Observations

2a. Nitrogen Dioxide Gas

• Brown gas (NO 2) evolves just above liquid level


N.B.: The reaction is faster when hot acid is used.

HNO 3(conc.)Zinc metal

Zn (s) + 4 HNO 3 (conc.) Zn(NO 3)2 (aq) + 2NO 2 (g) +2H 2O(l)

Brown gas

a) Conc. Nitric acid



Observations

2b. Nitrogen Dioxide Gas

• Brown gas (NO 2) appears at the upper part of thetest tube.


N.B.: colourless nitrogen monoxide forms which reacts withair oxygen to form brown nitrogen dioxide gas.

HNO 3 (dilute)Zinc metal

3Zn (s) + 8 HNO 3 (conc.) 3 Zn(NO 3)2 (aq) + 2 NO (g) +4 H 2O (l)

2NO (g) + O 2 (g) 2 NO 2 (g)

Brown gas

b) Dilute Nitric acid



Other reaction

Dehydrating property of conc. Sulphuric acid

a. Reaction with sugar

• Black spongy solid of carbon forms

C12

H22

O11

(s) 12 C (s) + 11 H2O (l)

Reason: H and O atomsare removed from sugar

molecules in the ratio 2:1.Water is formed andcarbon solid remains

comparing acids (dilute and strong)

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